Conventionally, various communication networks (e.g., optical networks) utilize various mechanisms to provide mesh switching, such as Optical Signal and Routing Protocol (OSRP), Automatically Switched Optical Network (ASON), Generalized Multi-Protocol Label Switching (GMPLS), and the like. In mesh switching networks, each node is aware of its own set of connections and the network topology (i.e., network elements and links), including the available bandwidth on each link, link admin weight or cost, and the physical diversity of links based on shared risk link group information. Based on this information, the node is able to calculate least cost restoration paths in the network which are diverse from the current route of a given connection. These restoration paths can either be computed in direct response to a failure event or pre-calculated in advance of the failure. In both cases, the calculated restoration path attempts to utilize network bandwidth that is both link and bundle diverse from the failed/current working path. These restoration paths may also be user-specified preferred or exclusive paths that the connection can take during failure or a mesh restoration event. Such paths are specified through the assignment of a designated transit list (DTL). One or more such restoration paths may be specified in a DTL set.
Conventional mechanisms suffer from the problem that an individual node does not have a global view of all the connections in the network. The node is fully aware of all connections that it originates and is responsible for calculating restoration paths for these connections. Each node in the network performs this calculation independently and multiple nodes may calculate a common restoration path for circuits affected by a given failure. In such a case, the bandwidth on this restoration path may become oversubscribed. At the time of failure, the first connections to attempt restoration on this route get the bandwidth on a first come, first serve basis. Additional connections which reflect the degree of oversubscription encounter crankback and the originating nodes are forced to calculate alternate restoration paths. As restoration events become larger, this process of bandwidth contention can recur through multiple iterations before all connections are fully restored. This process can lead to longer restoration times and increased signaling traffic in the network. The increase in signaling traffic can further delay the ability for routing messages to be propagated through the network. The routing messages are necessary to fully inform the originating nodes of affected connections on the actual bandwidth utilization of links in the network. Latency in the ability or routing updates to propagate through the network can further cause crankback to occur as connection attempts continue to be tried on links for which bandwidth is not available.
Another issue is that to ensure protection routes are valid at the time of failure, they must be calculated such that they are bundle diverse from all possible failures along the route of the connection. A failure may occur at any point along the route of a connection which can include multiple hops. Each hop is represented by a single link, but that link can further be part of a bundle of links that has a probability of simultaneous failure. For example, one link can be represented by a single wavelength in a dense wave division multiplexed (DWDM) span or a single fiber within a common conduit. In addition to the links which the connection actually traverses, any of the links which share a common shared risk or bundle identification must be excluded from possible restoration paths at the moment of failure. This can remove a significant amount of potentially available restoration bandwidth from the network on the probability that it is failed. At the time of failure, routing updates on the exact state of the network have not had an opportunity to propagate and potentially failed bandwidth is removed from consideration in an effort to speed restoration times by avoiding crankback. This can result in a higher cost path being selected for restoration than what might be available. It also can limit the number of available paths in the network to a smaller pool of diverse bandwidth which is more likely to become oversubscribed.
In conventional implementations, a DTL set can contain more than one restoration paths, but they are utilized as a hierarchy of choices. The same hierarchy is used regardless of which link in the end-to-end connection may fail. Such DTLs can be used to steer restoration away from oversubscribing bandwidth, but in a global restoration calculation, they must be assigned based the sum of all hops a given connection traverses and not focused to the actual link which may have failed. This can be used to the lower the probability of bandwidth oversubscription, but does not have the ability to eliminate it.